Recently, high-density integrated circuits (IC) and large-scale integrated circuits (LSI) which employ MOS transistors to process analog signals have been produced. An LSI having incorporated therein a CCD (charge coupled device) transversal filter for equalizing the waveform of video signal to remove the ghost mixed in a television signal is also one of the MOS analog LSIs. This transversal filter includes a circuit for controlling the gain of an amplifier analog signal to the input signal. The ghost mixed in a television signal has not only the same polarity as a video signal (desired signal) but opposite polarity thereto, and therefore in order to remove the ghost, it is necessary to properly control the gain of the non-inverted or inverted amplified video signal to the input video signal and thus to use two control signals therefor. In order to meet this requirement, an amplifier is necessary which is suited to be integrated and has one input and two outputs, that is, non-inverted and inverted output signals the waveforms of which are the same.
FIG. 1 is a circuit diagram of a well-known differential amplifier with a load of resistors. There are shown a power supply 1 (voltage=+V.sub.DD), load resistors 2 and 21, an output terminal 3 at which an inverted signal opposite in phase to an input signal is produced, an output terminal 31 at which a non-inverted signal in phase with the input signal is produced, a signal input terminal 4, a reference biasing source (voltage=+V.sub.BB) 41, load-drive elements, or N-channel MOS transistors 5 and 51 for amplifying the input signal, an N-MOS transistor 6 for a constant current source, and a gate biasing source 7 for the N-MOS transistor 6. The input terminal 4 is connected to the gate electrode of the N-MOS transistor 5. The anode of the biasing source 7 is connected to the gate electrode of the N-MOS transistor 6 and the anode of the biasing source 41 to the gate electrode of the N-MOS transistor 51.
The differential amplifier of FIG. 1 is excellent in the linearity of the amplification factor. To obtain a large voltage gain and to consume small power, it is necessary to select the load resistors to be of large value. Each of the resistors of large values needs a large area on the IC chip, and therefore the area of the chip must be large. Thus, a large-value resistor is not suited to be formed in an IC chip. This situation will be described below. FIG. 2 shows the DC input-output characteristic of the differential amplifier of FIG. 1. The abscissa shows an input voltage V.sub.in, the ordinate shows an output voltage V.sub.out, V.sub.01 is inverted output signal, and V.sub.02 is a non-inverted output signal. The input voltage V.sub.in at the intersection between the inverted and non-inverted outputs V.sub.01 and V.sub.02 is the input reference bias voltage (=+V.sub.BB). The actual usable range of the differential amplifier is the crossing portion with its vicinity of the outputs V.sub.01 and V.sub.02, the slopes of which show the gains of the amplifier. Since the magnitudes of the gradients of the outputs V.sub.01 and V.sub.02 waveforms are the same and constant, the output signal waveforms are analogous to the input signal waveform, that is, good linearity can be achieved. However, for large voltage gains, the value of the load resistors must be large.
The voltage gain, Av can be expressed by EQU Av=.+-.G.sub.m R.sub.L ( 1)
where g.sub.m is the mutual conductance of the drive MOS transistors, R.sub.L is the load resistance value, and the plus and minus signs, .+-. show non-inverted and inverted outputs, respectively. Since the value of the mutual conductance g.sub.m is substantially constant, depending on the drive MOS transistors used, the value of load resistor R.sub.L must be increased for large gains. Generally to obtain the gains of several dB or above, the load resistance value must be selected to be several K.OMEGA.. If the load resistors of such value are formed on an IC chip, a large area is occupied on the chip by the resistors and in addition the actual resistance is deviates greatly from the target value. Therefore, it is difficult to produce circuits having good characteristics, and, hence, large-value resistors are not suited to be used in an IC.
FIG. 3 is a circuit diagram of another differential amplifier using loads of N-MOS transistors instead of the load resistors. In FIG. 3, there are shown N-MOS transistors 22 and 23 for loads. The gate electrodes of the N-MOS transistors 22 and 23 are connected to the power supply 1. The differential amplifier of FIG. 3 can be small-sized because the loads used are not resistors. However, this amplifier has very poor linearity as compared with that of FIG. 1. The reason for this will be described below.
FIG. 4 shows the DC input-output characteristic of the differential amplifier of FIG. 3. The abscissa indicates the input voltage V.sub.in, the ordinate the output voltage V.sub.out, V.sub.03 the inverted output, and V.sub.04 the non-inverted output. The input voltage at the intersection between the outputs V.sub.03 and V.sub.04 corresponds to the input reference bias voltage (=V.sub.BB). As compared with the characteristic curves of FIG. 2, those of FIG. 4 are unsymmetrical with respect to the line drawn through the crossing in the direction of abscissa. In other words, since the gradients of the output curves V.sub.03 and V.sub.04 are not constant, i.e., since the ratio between the input and output voltages is not constant, the linearity is poor.
FIG. 5 shows a curve of the resistance characteristic of the N-MOS transistors for loads. The abscissa is the voltage V.sub.DS between the drain electrode and source electrode of each of the N-MOS transistors, and the ordinate is the on-resistance Ron-n thereof. When a signal is applied to the input of the differential amplifier of FIG. 3, the drain-source voltages of the load-drive MOS transistors 5 and 51 are changed, and as a result the drain-source voltages V.sub.DS of the load MOS transistors 22 and 23 are changed. As is apparent from FIG. 5, the on-resistances Ron-n of the load transistors are changed with the change of the drain-source voltages V.sub.DS. Therefore, the resistances of the load transistors are changed by the value of the input voltage. Consequently, the voltage gain A.sub.v is not constant from Eq. (1), and hence the lineality of the differential amplifier of FIG. 3 is inferior to that of the differential amplifier using the resistance loads in FIG. 1.